1. Field of the Invention
The present invention relates to a highly efficient encoding apparatus for encoding image signals having a relatively large data or information content such as high definition television signals.
2. Description of the Prior Art
The applicant of the present invention has proposed an adaptive encoding apparatus for obtaining a dynamic range, which is the difference between a maximum value and a minimum value, of a plurality of pixels contained in a 2-dimensional block and for adaptively encoding the pixel data of the block in accordance with the dynamic range. This apparatus is disclosed in Japanese Patent Laid-Open Publication No. SHO 61-144989.
Further, disclosed in Japanese Patent Laid-Open Publication No. SHO 62-92620 is an adaptive encoding apparatus for adaptively encoding data in accordance with the dynamic range of a 3-dimensional block of pixels formed from a plurality of frames. Moreover, disclosed in Japanese Patent Laid-Open Publication No. SHO 62-128621 is a variable length encoding method for varying the number of bits in accordance with the dynamic range so that the maximum distortion due to quantization remains constant.
The above mentioned encoding technique utilizing the dynamic range (hereinafter, named ADRC) can remove level redundancy and decrease the number of bits per pixel so that the amount of data to be transmitted can be compressed.
Further, a predictive encoding technique is known in which the value of a current pixel is predicted by means of the value of an encoded pixel and in which a predictive error, that is, an error between the true value of the current pixel and the predictive value, is transmitted. This predictive encoding technique enables the amount of data to be transmitted to be compressed.
In another highly efficient encoding technique known as a 2-dimensional DCT (Discrete Cosine Transform) technique, an image is divided into blocks, each block is orthogonally transformed, and the resultant coefficient data is quantized. The coefficient data are divided into a DC component and a high frequency component. Typically, the value of the DC component is relatively large, while the value of the high frequency component is substantially smaller. By assigning a particular number of bits to each component, the total number of bits can be decreased in comparison with the number of bits of the original data.
However, even if one of the above mentioned ADRC, DCT or predictive encoding techniques is used, the data compression ratio may not be satisfactory. Further, if the compression ratio is increased, the quality of the decoded image becomes poor.